On-frequency repeaters are known in the art for improving wireless services within defined regions of a wireless network, where signal levels would otherwise be too low for satisfactory quality of service. For example, within a building, or a built-up urban area, signal attenuation, shadowing by buildings and/or hills; noise generated by various radio frequency sources, and multi-path effects can seriously degrade the quality of desired RF signals. In some cases, a wireless network provider may install a repeater in order to improve service in a region lying at an edge of the coverage area serviced by a fixed station, thereby effectively extending the reach of the base-station. On-frequency repeaters are characterized by the fact that the input and output signals (in either the uplink or downlink path directions) have the same frequency.
As may be seen in FIG. 1a, an On-frequency repeater 2 typically comprises a donor antenna 4 which “faces” a fixed station 6 and enables bi-direction RF signal traffic between the repeater 2 and the fixed station 6; a coverage antenna 8 which faces a mobile communications device (MCD) 10, such as a cellular handset; and an amplifier unit 12 connected between the donor and coverage antennas 4 and 8. The donor and coverage antennas 4,8 are passive devices, and are coupled to the amplifier unit 12 via suitable lengths of co-axial cable. This architecture suffers a disadvantage in that the amplifier unit 12 must compensate for line losses within the co-axial cables in addition to signal path losses (to/from the fixed station 6 and to/from the MCD 10), which increases the performance requirements, and thus the cost, of the amplifier unit. Compounding this problem is the fact that the high-gain components needed to deliver the required amplifier performance creates significant interference and cross-talk within the amplifier unit 12. Solving these issues further increases the cost of the repeater system.
Applicant's co-pending U.S. patent application Ser. No. 09/809,218, which is incorporated herein by reference, teaches a distributed repeater architecture. As shown in FIG. 1b, the repeater system is divided into a donor unit (DU) 14 and a coverage unit (CU) 16, each of which includes an amplifier block 18 and 20 integrated with a respective one of the donor and coverage antennas 4 and 8. A co-axial cable 22 connecting the two units 14 and 16 provides an RF path for up-link and downlink signal traffic, and also a DC power supply line so that, for example, the amplifier block 18 of the DU 14 can be powered by a power supply (not shown) of the CU 16. Typically, the amplifier block 18 of the DU 14 provide wide-band signal amplification to overcome signal losses in the co-axial cable 22. The CU amplifier block 20 provides more sophisticated signal processing functions, such as gain, power and system management, and oscillation control functions, some or all of which may be governed by a micro-controller 24 operating under software control.
As described in U.S. patent application Ser. No. 09/809,218, dividing the amplification unit 12 of the conventional repeater between the DU 14 and the CU 16 has an advantage that it enables lower-performance—and thus lower-cost—components to be used, without sacrificing overall repeater performance.
A limitation of the system of FIG. 1b is that integration of active electronics 20 and the coverage antenna 8 within a single coverage unit 16 imposes some limitations on the placement of that unit. In particular, the coverage unit 16 should preferably be positioned at a location that is most favourable from the point of view of optimizing the coverage area of the CU antenna 8. However, this location may not be conveniently close to a source of electrical power, for example. In addition, the close integration of the coverage antenna 8 and active electronics 20 within the CU 16 may impose undesirable limitations on the maximum permissible coverage area that can be served by the repeater system.
Accordingly, a low cost, extensible repeater architecture remains highly desirable.
A further limitation of the system of FIG. 1b is that oscillation may occur if the gain between the donor antenna and any individual coverage node is less that the net gain between the antennas, in either the uplink or downlink direction. The difference between the repeater gain and antenna isolation is known as the stability margin of the system.Stability Margin=Antenna Isolation−Net Repeater Gain
If the stability margin is <0 dB, the repeater system will oscillate at one or more frequencies, generating interference in the host communications network. Even if the system does not oscillate, operation at point where the stability margin is low (<3 dB, for example) may cause the noise output of the repeater to increase above its normal value. In the uplink direction, this may cause desensitization of the base-station, with consequent reduction in the coverage area of the base-station. It is therefore highly desirable that a distributed repeater system such as that shown in FIG. (1b) should be provided with the means to operate with a constant and adequate stability margin, as presented in applicants co-pending U.S. patent application Ser. No. 10/299,797 which describes a means of monitoring and maintaining the stability of an on-frequency repeater comprising a Donor Unit and a Coverage Unit, based on the use of a Coverage Area Signature (CAS).